Mysteron could be anyone, except Axl. There are probably so many important people inside GNR that we've never heard of. Maybe Mysteron is one of them, or not.
Actually, I know who Mysteron is. Or, maybe I just heard a really good theory somewhere along the line. Either way, somebody told me about his connection with the band at some point..........
But maybe it's all like, shh, and it's a secret.